Here is one member's experience:
My son was a young 2 when my twins were born. I ended up using a double stroller with a buggy board attached for my oldest to ride on.
Here is where I bought it.
It was $90.
Here is more information on it.
There is a list of strollers on this website that work with it. I ended up getting an Inglesina double stroller (very lightweight) off Craigslist. It wouldn't work with the double Combi stroller I had.
My son loves to ride on the board. It is a bit louder, but so worth it to me. He doesn't want to walk at all now though. It is very easy to take on/off (I'm talking seconds here) if it is just the twins. It is very lightweight too. I thought about the Valco with the extra seat, but this was so much easier, less expensive, and he just likes it so much.
My second son was also 18 months when our twins were born. We did a few different things on our outings that I'm happy to share with you below.
We got the Combi double stoller when the twins were born -- it's a side by side, fairly lightweight (more of the umbrella stroller style), and we still use it (they are now 2). We've gotten tons of mileage out of it and it actually folds up into "4ths" so we easily keep it in our van always and we've made several airline trips with it as well. It's definitely been worth the money and I don't have trouble getting in through doorways,etc. I do have trouble in some clothing stores that have their racks packed in so tightly that there isn't room for much more than a standing person to walk through but I just try to avoid those stores when I have the kids. I don't typically have problems getting through the main aisles, though.
The only thing you should be aware of is, it does not hold 2 infant seats but it does have shoulder straps along with the lap belts, reclines, and will hold 1 infant seat. Combi does make infant seats to fit in their stroller but even so, only one will fit at a time. Having had 2 boys already going through the infant seat stage, I knew that stage would not last long and my husband and I decided it would be better to just invest in a stroller we liked and would prefer using over the long haul. So, we dealt with the temporary inconvenience of only being able to use the Combi stroller with one infant seat and looking back, it didn't seem that bad.
All that being said, here's what we did on our outings with the boys:
1. Used the Combi stroller with one twin in the infant seat, the other twin in our Bjorn carrier and our 18 month old in the other side of the stroller.
2. Used the Combi stroller with one twin in the infant seat, the other twin out of the infant seat, reclined, and strapped in to the stroller and our 18 month old in a single stroller. I could only manage this if someone was with me and pushing the other stroller (often, my then 5 yr old).
3. We also used our plastic Radio Flyer wagon which worked perfectly with both infant seats placed side by side. The little notch on the bottom of the Graco snug ride seats fit over the edge of the wagon so I felt they were fairly secure. I managed many zoo trips and neighborhood walks on my own this way because I could pull the wagon and push our 18 month old in a stroller at the same time.
It wasn't long before time passed and our 18 month old got to the point of not wanting to be in a stroller anymore and preferred walking on his own (or chasing after his older brother) and I just had the twins in the stroller. I know there are a lot of wonderful choices and some really nice strollers out there, but for us the Combi fit in our budget and we're still very happy with it. Feel free to email me with any additional questions and I'm hoping you'll get some additional input from all the other moms who have also been through this.